1. Field of the Invention
The present invention relates to a magnetic particle and to a method of preparing the same. More particularly, the present invention relates to a magnetic particle that has magnetic characteristics suited to magnetic recording and that can be employed in a particulate magnetic recording medium, and to a method of preparing the same.
2. Discussion of the Background
In widely employed magnetic recording media, such as video tapes, computer tapes, and disks, the smaller the particles of magnetic material, the higher the SNR becomes for a given content of magnetic material in the magnetic layer. This is advantageous for high-density recording.
However, as the size of the magnetic particles decreases, superparamagnetism ends up occurring due to thermal fluctuation, precluding use in a magnetic recording medium. By contrast, materials of high crystal magnetic anisotropy have good thermal stability due to a high potential for thermal stability. Accordingly, research has been conducted into materials of high crystal magnetic anisotropy as magnetic materials of good thermal stability. For example, high crystal magnetic anisotropy has been achieved by adding Pt to a CoCr-based magnetic material in hard disks (HD) and the like. Investigation has also been conducted into the use of CoPt, FePd, FePt, and the like as magnetic materials of higher crystal magnetic anisotropy. Further, magnetic materials containing rare earth elements, such as SmCo, NdFeB, and SmFeN, are known to be magnetic materials that do not contain expensive Pt, that are inexpensive, and that exhibit high crystal magnetic anisotropy (referred to as “Technique 1”, hereinafter).
Although materials of high crystal magnetic anisotropy afford good thermal stability, an increase in the switching magnetic field necessitates a large external magnetic field for recording, compromising recording properties. Accordingly, the Journal of the Magnetics Society of Japan 29, 239-242 (2005), which is expressly incorporated herein by reference in its entirety, describes attempts that have been made to reduce the switching magnetic field by stacking a soft magnetic layer and a hard magnetic layer formed as gas phase films on a nonmagnetic inorganic material to produce exchange coupling interaction (referred to as “Technique 2”, hereinafter).
In metal thin-film magnetic recording media such as HD media, a glass substrate capable of withstanding high temperatures during vapor deposition is normally employed as the support. By contrast, particulate magnetic recording media affording good general-purpose properties and employing inexpensive organic material supports have been proposed in recent years, and are widely employed as video tapes, computer tapes, flexible disks, and the like. From the perspective of maintaining the general-purpose properties of such particulate media, it is difficult in practical terms to employ a magnetic material in which expensive Pt is used. Thus, the use of a magnetic material comprising a rare earth element such as in Technique 1 is conceivable. However, as set forth above, improvement of recording properties is required for magnetic materials of high crystal magnetic anisotropy.
Accordingly, the application of Technique 2 to particulate magnetic recording media is conceivable to achieve both thermal stability and recording properties. However, in Technique 2, the support is exposed to high temperatures during gas phase film formation. Thus, it is difficult to apply this technique to nonmagnetic organic material supports usually employed in particulate magnetic recording media because these supports are of poorer heat resistance.